Hello all. I had a problem that I reported last year on this forum. The 3.8 S Type backfires and splutters when i push the accelerator pedal down when the engine is colder than operating temp. Once it reaches its operating temp its fine but that 10 minutes or so while its getting up to temp are a real bummer. The colder it is the more it splutters and backfires. This decreases proportionally to the engine temp until there is none when fully warmed up.
The twin SUs have been rebuilt (ouch) and the car has been tuned (at operating temp). I have tried enriching and leaning off the mixture with no real difference. I am convinced there are no air leaks. I am convinced the electricals are ok.
So I am after ideas. One that has been suggested is the oil in the dashpots. I can see how the oil would get thinner as it got hotter but would dashpot oil that is too thick have this effect?

 

Similar problem here with a 3.4s Auto. When cold especially in the winter even with the AED in full operation if I try and accelerate the engine dies to below idle and sometimes stops. I have to feather the throttle and get the revs up before it pulls which is not easy with an auto. I know start the car on the drive and wait until the car is warm before proceeding. I have rebuilt the carbs, colour tuned the carbs, set the timing, adjusted the slow running jets, adjusted the floats, adjusted the AED needle but still the problem persists. Would love to have some feed back and a solution but in the meantime I just get the car up to running temp on the drive before I take it out. I think it is a problem with the AED as once warm and the AED turns off the problem goes away but not sure what it could be.

 

Hi Rob. I am convinced my AED is working correctly. Is there any chance it could be the viscosity in the SU dampers?

 

Glyn is the man to ask about oil viscosities. My SU dampers are topped up but I only use standard 20/50 Castrol engine oil in them. If there is a correct oil to use Glyn should know.

 

The dampers run very cold due to vapourisation of fuel/venturi effect. Only SAE 20W in the dampers. Listen to SU even if you have to buy their genuine oil. ATF is too light. 20W monograde is readily available in some markets & difficult in others. People make the mistake of thinking the dampers run at approx underbonnet temperature. They don't.

Is your AED needle set up correctly for your ambient conditions? You might not be rich enough.

The dampers on the Jag don't seem to go below freezing as they did on some SU fitted Minis.(carb warmers were available for Minis). I have not managed to rig up a proper measuring device on my dampers but have caught them at 6 deg C with a jury rigged Fluke touch thermometer.

 

The automatic choke is set too lean, that's the problem.
There is an adjusting nut in the middle where the air is drawn into the choke.
Screwing the nut in leans the choke out and screwing it out makes it rich.

Be careful, one can easily make it too rich.
If it starts and runs, but stalls when you try to pull away, it's about right, so no more then 3 flats of the nut.
Keep in mind that it may be too rich for summer if you adjust it to start and drive in the winter.

There is nothing wrong with the carburetors, that is the way the choke works, it's not a very good design.

Cass, if you have adjusted the needle, as you said, then keep turning the nut out until the stalling problem stops, but don't go too far or you will flood it.
It's just the way it is.
It's not the best choke system on the planet, it can't adjust itself for ambient temperature and humidity.
It would be great if it was like a fuel injected vehicle or even a North American choke with a bi-metal strip and choke flap, but it isn't.

 

I know you have the SU book I suggested but here.

https://sucarb.co.uk/technical-choke...ary-enrichment

 

Thanks for the feedback so far but we need to forget about the AED - it is not the AED
The issue occurs all the way from 0 degrees up to 80 degrees when it id fully warmed up. The ARD turns off at 35 degrees.
So, if the oil in the dampers is too thick what are the symptoms please.
If the oil in the dampers is too this what are the symptoms please?
cheers

 

Have you checked the ignition timing? Could it be too advanced?

 

The oil in the damper tubes won't affect how the car starts or cause it to start poorly.
What the damper tubes are there for, and the oil, is to keep the car from hesitating from a stop when you accelerate.
The damper tubes are there to keep the pistons from suddenly rising too quickly when the butterfly valves are open when the accelerator pedal is depressed.
Using too thick of oil, or to thin, or even no oil at all is not your problem and won't show any real symptoms other then a slight hesitation from a start. You won't be able to tell using thin or thick oil.
Red automatic transmission oil works well in case you're wondering.

A too far advanced ignition would cause the engine to kick back when it's started and it would do it consistently all the time, an engine kicking back because it's getting the spark too soon, sounds like the battery is going dead when started from hot engine.
If it's so far advanced, it would knock and ping going down the road, pinging sounds like small ball-peen hammers pounding on your piston's, essentially the fuel is being ignited so early, it's trying to push the pistons in the wrong direction, hence the pinging sound.

Wouldbeowner, humor me, the next time the car doesn't start even after it's fully warmed up, pull the wire off the Otter Switch where the thermostat housing is, (be sure and pull the correct wire off, don't pull the wire off for the temperature gauge) or even better, once the car is fully warmed up, pull the wire of the Otter Switch, so you know it is off.
The Otter Switch is not very accurate, even the new ones; the only way to be sure is to pull the wire off.
This has to be taken off the table, just because the Otter Switch is supposed to go off at 35 degrees or just because it's new, doesn't mean to say it's working properly and turning off at 35 degrees.
It's a very unreliable piece of equipment, that's why many here have replaced it with a toggle switch somewhere under the dash.

Wouldbeowner, in your first post you say the problem goes away only after the engine is fully warmed up, then in post #8, you say it happens from 0 degrees to 80 degrees.
Quote "proportionally to the engine temp until there is none when fully warmed up", and then in post 8, quote, "The issue occurs all the way from 0 degrees up to 80 degrees"

If the Otter Switch is not shutting off properly, it will produce the symptoms that you describe, it can't fully open the solenoid when the engine is running, there is too much vacuum for the solenoid to over come and open the choke. However, when one try's to feather the accelerator pedal, this may allow the Otter Switch to partially open the solenoid and allow some fuel to get in when it's not wanted.
And if one tries to tune the engine under these circumstances, it would not be possible to tune the engine.

Another problem with the Otter Switch is, the car may start fine when the switch is on and the choke solenoid opens as it should. You drive for 10 minutes and the engine is warm enough to run with the choke off.
Now 10 minutes of driving may be long enough to warm the combustion chambers so there is no choke that is necessary, but the coolant is not really warmed up yet, so the Otter Switch still thinks the engine needs to be choked, when in fact it doesn't. So in this situation, the car gets choked when it doesn't need it, this will also cause you hard starting problem; a manual toggle switch does a way with the Otter Switch and solves this problem.

Sorry Glyn, I'm going to disagree with 20 weight oil in the damper tubes, unless it's 100% synthetic, 20 weight non-synthetic oil is too thick when cold.
As for ATF, I've always used that because it stays thinner when the fall weather gets cold, but I suppose a full synthetic 20 weight oil would work.
Personally the oil in the damper tubes is just not that critical, when everything thing else is tuned properly, the oil in the damper tubes just isn't going to affect the proper over-all performance of the engine because it does so very little when one pulls away from a stop.

Cass, depending on how rich you have the AED, letting the car warm up in the drive way, usually isn't sufficient enough to properly warm the combustion chambers.
I did an experiment once to find out just how long I had to leave my car warm up sitting still from a cold start, it took at least 20 minutes, 1/2 hour was even better.
I have mine set up for a heated garage and that mimics summer time temperature's when I drive it.
However a problem arises if I drive in the fall and it gets parked outside when the temperature drops below the environment of my garage.
It won't start, I have to turn the adjusting nut out at least 2 to 3 full turns to enrichen the choke, but if I leave it that way once it's parked in the heated garage, it floods, it's a **** poor design of a choke having the need to keep changing its mixture nut depending on the temperature that the car is parked in.

I've never found a happy medium where I can set it leave it.

 

Jeff. The genuine 20W from SU is now fully synthetic. SU have recommended 20W forever. I think they know their carbs. ATF is generally too thin unless you are in the arctic (below a 10W unless Types F & G which is just at the bottom of the 10W range). What most people don't realise is how cold the dampers run. When in doubt. Listen to the OEM.

NSW Australia has relatively mild conditions like SA & especially Cape Town. I have lived in Sydney. I have no issue at all setting up the AED for normel running from cold here.

The oil in the dampers really just promotes a throttle pump effect on acceleration to prevent hesitation or stumble.

The OP's symptoms are of running too lean. Air leak, false air (but the carbs have just been rebuilt) or simply tuned too lean. Unless there is electrical breakdown somehere (could well be a bad coil from symptoms) or inadequate fuel pressure. (But then I would expect loss of performance when making high demands of the engine as well fully warmed up.)

 

Thanks Glyn. That is what I needed.
So, what are the symptoms if the dashpot oil is too thin?
What are the symptoms if the dashpot oil is too thick?
Do the SUs on different makes of car use different viscosity dashpot oil please?
TIA

 

Glyn, there are too many variables and unanswered questions here to make a proper diagnosis.
All I know is that the oil in the damper tubes isn't that critical and certainly what ever the OP puts in there, isn't going to effect or help his problem.
I've used ATF and thicker motor oil, and if there is a difference in how the car runs or behaves, I can't detect the difference.

The temperatures and humidly on the South West Coast here in Canada change drastically and I have to lean or enrichen the choke to suit.

When I hear "rebuilt carburetors" I cringe a little, who rebuilt them, did they know what they were doing, just because one puts a kit in them doesn't mean to say it was done properly ?
The butterfly shaft may be worn out, or the body of the carb is worn, so even if a new shaft was put it, it may be leaking around the shaft.
Just because someone payed a lot of money to have them rebuilt, doesn't mean to say it was done properly.

I don't agree with some of Joe Curto's method's where he uses a drill press to ream out the butterfly shaft in the carburetor, he's holding it by and, he should be using a milling machine.

I'm not going to add anything else here, we'll see how far the OP gets.

 

Hello all "op" here. I am not sure everyone is looking at all the symptoms.

The misfiring goes away 100% by fully up to operating temp. It is a bit better a couple of minutes after startup, a bit better again a couple of minutes after that and it gradually gets better until it is running properly when fully warmed up.
It is 100% proportional to the temperature of the car.
I am pretty confident the carbs have been rebuilt properly, I am confident the aed is ok.
I am pretty confident it isnt air leaks - if it were air leaks then why would it be so 100% predictable and proportional to temperature?
I believe the mixture is correct based on the colour of the exhaust and, more importantly, the state of the spark plugs.
I believe the timing is correct.
I will go out and check the amount of oil in the damper

 

OK, thanks.
Given the fact that it runs well after it's fully warmed up, the AED is simply set too lean.
Even at the best of times, the AED is not the most precise instrument, but enriching it should (will) fix your problem.
Be carful you don't make it too rich, or the car will flood, one or two flats of the adjusting nut at a time.

 

Thanks Jeff but the AED turns off at 35%C (as it should) this occurs about 1 minute into the startup procedure BUT the misfiring continues for another 5 or more minutes while the engine continues to warm up.

IE the AED is already out of the circuit but the problem continues.

Anyway, I just checked the dampers and they were bloody well near empty. I have topped them up and it is certainly better - not 100% but now I will try enriching mixture a little or looking at timing just to make sure.
I will keep you good people informed as to what happens.
Cheers and Thanks

 

All SU Carbs use SAE 20. Never seen another another recommendation be it a Volvo, MG, Aston Martin or Mini. It's an SU spec.

Too thin and you will get spit back & stumble on hard pull away or quick throttle opening. Too thick and the throttle feels "dull" as you get a burst of over rich running as the pistons are slow to rise. On a Jag S Type you are inclined to see a burst of black/brown over rich smoke from the exhausts that then clears with over thick oil. I have mine running clear on both pipes.

I do not think dashpot oil viscosity is the major cause of your problems.

These cars will never drive like a fuel injected unit cold. But your indications are generally too lean from what you explain & could also be a duff coil. Without personally driving the car these things can be difficult to diagnose.

How are the brakes? No air leaks in the vacuum circuit? Vacuum advance pipe & elbows all good on the distributor etc.?

One accepts that the Otter switch very approx cut's the AED at 35 deg C but how are you setting up the AED needle? I'm highly suspicious of the AED setting.

The plugs should be pitch black after a cold start until they have been running in the self cleaning range for quite a way fully warm.

Good luck.

 

This is not about the AED turning off or on, it's adjusting while it's on and that is done with number 48, this is how to adjust the mixture so the car won't stall, anticlockwise to make it rich, and clockwise to lean it out.
Yours needs to be made rich, I would start with 2 or three flats counterclockwise.
This has nothing to do with setting the mixture on the carbs themselves, and that sounds correct. don't touch that.

 

 

Thanks Glyn and Jeff - I will have a fiddle.
Glynn, I have not been able to find any air leaks any where in the intake circuit.
I think I will increase the richness of the AED as per Jeff then go back to scratch with the timing and then the mixture of the carbs themselves.
One thing I have learned from all this is that no matter what a mechanic tells you about how much they know and "these are just like any other cars" dont believe them.